1. Field of the Invention
The present invention relates generally to a digital broadcasting system, and in particular, to a method and apparatus for performing a handover in a digital broadcasting system.
2. Description of the Related Art
A Digital Audio Broadcasting (DAB) system is a digital broadcasting system for audio and data services, which has been approached in European, aiming at digitalization of the existing AM and FM broadcastings. A frame structure of the DAB system will be described hereinbelow.
FIG. 1 shows a frame structure of a general DAB system. A DAB frame 101 is composed of a synchronization channel 102, a Fast Information Channel (FIC) 103, and a Main Service Channel (MSC) 104. The DAB frame 101 is composed of 1 Null symbol and 76 symbols. A first Orthogonal Frequency Division Multiplexing (OFDM) symbol is a Null symbol and a second OFDM symbol is a Phase Reference Symbol (PRS) symbol. Because there is no transmission signal in the first Null symbol, energy for a Null symbol interval is considerably lower than that of other symbol transmission intervals. Therefore, a receiver can find a frame start time by detecting the Null symbol interval using an energy detection process. The second symbol, a PRS symbol, transmits phase information for data demodulation. These two symbols constitute the synchronization channel 102, and the synchronization channel 102 corresponds to the two leading symbol intervals in the DAB frame. The synchronization channel 102 is followed by an interval of the FIC 103, and 3 symbols are used for transmitting the FIC. The remaining 72 symbols are used for transmitting the MSC 104.
The FIC 103 is composed of Fast Information Blocks (FIBs) 105, and the number of FIBs 105 transmitted through one FIC 103 differs according to transmission mode. A total of 4 transmission modes are possible: the number of FIBs is 12 for Transmission Mode 1, 3 for Transmission Mode 2, 4 for Transmission Mode 3, and 6 for Transmission Mode 4. The FIB 105 has a length of a total of 256 bits, and is composed of 240-bit FIB data 106 and 16-bit Cyclic Redundancy Check (CRC) 107. The FIB data 106 is composed of several Fast Information Groups (FIGs) 108. The FIG. 108 is divided into a FIG header 109 and a FIG data interval 110. The FIG header 109 is composed of a FIG Type 111 indicating a type of the data transmitted in the FIG data interval 110, and a FIG Length 112 indicating a length of the FIG data interval 110. The FIG. 108 is transmitted through the FIB data 106, and when the total length of the transmission FIG 108 is not equal to a length of the FIB data 106, an End Indicator 113 is used to indicate an end of the valid data interval and the remaining interval is padded with ‘0’. The End Indicator 113 is composed of 8 bits, all of which are ‘1’.
The MSC 104, an interval where transmission data is situated, is composed of Common Interleaved Frames (CIFs) 114, and the number of CIFs 114 differs according to transmission mode. The number of CIFs transmitted over one MSC is 4 for Transmission Mode 1, 1 for Transmission Modes 2 and 3, and 2 for Transmission Mode 4. The CIF 114 has a 55296-bit length, and is composed of subchannels 115. When the 55296-bit CIF cannot be fully filled with the transmission subchannels, the remaining interval is padded with ‘0’.
Information transmitted over the FIC includes an FIC data service signal used for transmitting traffic information or emergency messages, Multiplex Configuration Information (MCI), Service Information (SI), etc.
The MCI transmits position and length information for each of the subchannels constituting the MSC, and information on a channel coding rate of each subchannel. In addition, the MCI transmits a list of services transmitted through one ensemble. Further, in an occasion where several services are transmitted through one ensemble, the MCI transmits the connection relationship information indicating to which service components each of the services is connected. Moreover, in an occasion where each of the services is connected to one or several service components, the MCI transmits the connection relationship information indicating through which subchannel each of the service components is transmitted. Besides, when the multiplexed information is reconfigured, the MCI transmits the reconfigured information.
The SI indicates information on each service, which is not included in the MCI. The SI includes information related to the services included in the currently transmitted ensemble, such as service component information, time and country information, program information, etc. In addition, the SI includes information on other ensembles and services, such as Frequency Information (FI), Transmitter Identification Information (TII), Other Ensemble (OE) information, Service Linking information, Region Definition information, Local Service Area information, etc.
The FIC data service signal refers to a signal of the service that transmits data rather than the control information through the FIC, and types of the FIC data services are classified into Paging, Traffic Message Channel (TMC), Emergency Warning Systems (EWS), etc. A channel where the data service is transmitted through the FIC is called a Fast Information Data Channel (FIDC).
The MCI, SI and FIDC are transmitted through a data part of the FIG, and a header of the FIG is set to a different value according to the data transmitted in the FIG data part. Therefore, the receiver can determine a type of the data transmitted through the header in the FIG data part. The FIG is variable in length according to the amount of transmission data, and the information on a length of each FIG can also be determined through the FIG header. The FIGs of several types are multiplexed, forming one FIC.
The MCI and SI are the information that the receiver should know to receive a DAB signal, demodulate the received DAB signal and provide the demodulated signal to the user. The receiver can demodulate the data transmitted through the MSC for each individual service using the MCI and SI, and then provide the service selected by the user.
The user uses the DAB service either in a fixed position or in a moving environment. When the user uses the DAB service in a fixed position, a terminal of the user is located in a constant distance from a DAB transmitter that transmits the DAB service signal, so a variation in received signal strength with the passage of time is not significant. However, when the user uses the DAB service while on the move, a distance between the user's terminal and the DAB transmitter undergoes a change. Therefore, the signal that the terminal receives from the DAB transmitter changes in its received quality with the passage of time.
The DAB transmitter transmits the above-described DAB frame, and the DAB frame transmitted by the DAB transmitter is called one ensemble. The ensemble, a signal of the DAB transmitter, is limited in the area where the signal can reach, according to transmission power of the transmitter. The area where the signal of the transmitter can reach is called a service area of the transmitter. In addition, the area where the receiver can receive the arrived ensemble is called an ensemble service area. In the case where the ensemble of the transmitter is transmitted from one transmitter, the ensemble service area is identical to the service area of the transmitter. However, the ensemble may differ for every DAB transmitter, or several DAB transmitters may transmit the same ensembles. Therefore, when several DAB transmitters transmit the same ensembles, the ensemble service area is not identical to the transmitter service area.
FIG. 2 shows an ensemble service area where there are four different DAB transmitters. Of the four transmitters, a DAB transmitter #1 201 transmits an ensemble A, a DAB transmitter #2 202 transmits an ensemble B, a DAB transmitter #3 203 transmits the ensemble A, and a DAB transmitter #4 204 transmits the ensemble A. Each of the DAB transmitters has its own transmitter service area. Each of the transmitter service areas refers to the area where a signal of the corresponding transmitter can reach, and as shown in FIG. 2, the transmitter service area is formed circularly centering on the corresponding transmitter.
Of the DAB transmitters, the transmitter #1 201, the transmitter #3 203 and the transmitter #4 204 transmit the same ensemble A. That is, although the 3 DAB transmitters have different service areas, the receiver can receive the same ensemble A in all of the service areas of the three DAB transmitters. Therefore, the ensemble service area where the receiver can receive the ensemble A is an area 205 including all of the service areas of the DAB transmitter #1 201, the DAB transmitter #3 203, and the DAB transmitter #4 204.
When a terminal 206 currently receiving the ensemble A in the service area of the DAB transmitter #1 201 moves to the service area of the DAB transmitter #3 203, the terminal 206 can continuously receive the ensemble A even though the DAB transmitter transmitting the ensemble A has changed from the DAB transmitter #1 201 to the DAB transmitter #3 203. The terminal 206 belongs to the service area of the DAB transmitter #1 201 when it is located in a position A, but the terminal 206 belongs to the service area of the DAB transmitter #3 203 when it moves to a position B. However, because the signals of the two transmitters are the same ensembles, the terminal 206 has no problem in receiving the ensemble A and providing the same service to the user.
However, if a terminal 207 currently receiving the ensemble A in the service area of the DAB transmitter #1 201 moves to the service area of the DAB transmitter #2 202, the terminal 207 can no longer receive the ensemble A for the following reason. When the terminal 207 is located in a position C, it is located in the service area of the DAB transmitter #1 201. However, when the terminal 207 moves to a position D, it is located in the service area of the DAB transmitter #2 202, which transmits the ensemble B rather than the ensemble A. Therefore, the terminal 207 cannot receive the ensemble A in the position D, so the existing service being provided to the user by the terminal 207 is interrupted.
As shown in FIG. 2, a variety of DAB transmitters exist in the DAB system, and in the environment where the terminal receiving the signal of the DAB transmitter moves from place to place, there is a possible situation where the terminal leaves the service area of the current DAB transmitter and moves to the service area of the different DAB transmitter. In a case where the terminal has moved between the service areas of the DAB transmitters, if the DAB transmitters transmit different ensembles, the terminal interrupts the currently received service.
However, the ensemble is composed of more than one service because several different services are multiplexed in the ensemble before being transmitted. The different services may include audio service, video service, data service, etc.
FIG. 3 shows service multiplexing in an ensemble according to the prior art. One ensemble 301 may include more than one service. The ensemble 301 will be called an ensemble A. The ensemble A has more than one service 302. The ensemble A 301 has 3 services 302 of Alpha-1 radio, Beta radio, and Alpha-2 radio. Each of the services 302 is composed of more than one service component 303. For example, Alpha-1 radio is composed of 3 service components of ‘Audio-1’, ‘Alpha-TMC’ and ‘Alpha-SI’. Beta radio is composed of 2 service components of ‘Audio’ and ‘2nd-ry Audio’, and Alpha-2 radio is composed of a total of 4 service components of ‘Audio 1’, ‘Audio 3’, ‘Alpha-TMC’ and ‘Alpha-SI’. Each of the services has a primary service component: Alpha-1 radio has the service component ‘Audio 1’ as a primary service component, and Beta radio has the service component ‘Audio 2’ as a primary service component. Alpha-2 radio has the two service components ‘Audio 1’ and ‘Audio 3’ connected to a switch, one of which becomes a primary service component. That is, unlike Alpha-1 radio having only one service component as a primary service component, Alpha-2 radio changes in its primary service component with the passage of time.
Each of the service components 303 is transmitted with one subchannel 304. The subchannels 304 used for transmitting their associated service components constitute an MSC, and the positions of the MSC where the subchannels 304 are transmitted can be determined depending on the MCI as described in connection with FIG. 1.
As shown in FIG. 3, various services are multiplexed in one ensemble. Therefore, when there are different ensembles of an ensemble A and an ensemble B, the same service may exist in the two different ensembles. If there is any same service in the different ensembles, there is a possibility that the terminal can continue the same service even though it receives the different ensembles.
The terminal can perform a handover process so that it continues the same service even though it receives different ensembles, and the terminal makes a spontaneous decision for handover based on strength of a received signal. However, the strength of the received signal may decrease, not only because the terminal is located at the boundary of the broadcast network, but also because even though the terminal is located in the vicinity of the broadcast transmitter, it cannot receive signals due to obstacles or as it enters a shadowing area. Therefore, if the terminal determines whether to start the handover process simply depending on only the strength of the received signal, it may encounter a handover error.